Optical articles from curable compositions

ABSTRACT

A curable composition is provided comprising an oligomer having a plurality of pendent and/or terminal ethylenically unsaturated, free-radically polymerizable functional groups, a free-radically polymerizable crosslinking agent, and/or a diluent monomer, and a photoinitiator. The composition, when cured, is non-yellowing, exhibits low shrinkage and low birefringence making it suitable for many optical applications such as optical lenses, optical fibers, prisms, light guides, optical adhesives, and optical films.

FIELD OF THE INVENTION

The present invention provides oligomeric compositions containingoligomers that are readily cured to produce optical articles andcoatings.

BACKGROUND OF THE INVENTION

Optical materials and optical products are useful to control the flowand intensity of light. Examples of useful optical products includeoptical lenses such as Fresnel lenses, prisms, optical light fibers,light pipes, optical films including totally internal reflecting films,retroreflective sheeting, and microreplicated products such asbrightness enhancement films and security products. Examples of some ofthese products are described in U.S. Pat. Nos. 4,542,449, 5,175,030,5,591,527, 5,394,255, among others.

Polymeric materials have found a variety of uses in optical articles andare widely used in place of such articles made from ground glass becausethe former are light in weight and inexpensive to produce. Carbonatepolymers, for example are characterized by excellent clarity, resistanceto discoloration, high strength, and high impact resistance. However,thermal polymerization of monomers to form polymers is generallyaccompanied by high shrinkage during cure (e.g., from 11 to 20%) andextended curing time (e.g., from 5 to 16 hours or more). The highshrinkage levels create difficulties in the production of precisionoptics (such as lenses or prisms) from this material, particularly inthe production of articles having larger thicknesses or largedifferences in thickness between the center and edges of the article.The extended cure times tie up production facilities and lead toinefficient utilization of the dies in which the articles are molded.Also, the thermal cure cycle used to polymerize the monomer consumeslarge amounts of energy and undesirably thermally stresses the dies.

Optical products can be prepared from high index of refractionmaterials, including monomers such as high index of refraction(meth)acrylate monomers, halogenated monomers, etc., and other such highindex of refraction monomers that are known in the optical product art.See, e.g., U.S. Pat. Nos. 4,568,445, 4,721,377, 4,812,032, and5,424,339. Some of these polymers may be advantageously injectionmolded, but such molding operations lead to high birefringence in theresulting article, and a subsequent annealing step may be required.Further, poly(methyl methacrylate) polymers tend to be moisturesensitive, and will swell on exposure to moisture or humidity, furtherleading to birefringence.

Several disclosures are related to optical coatings, which are generallyless than two mils (50.8 micrometers) thick. They fail to describe ifthose compositions would have a desired balance of useful propertiessuch as low polymerization shrinkage, low viscosity, absence ofcoloration, high hardness, resistance to stress cracking, moisture orhumidity sensitivity and low birefringence necessary in the productionof precision optical components such as lenses, including Fresnellenses, and prisms. Additionally, they fail to teach how to obtainresins providing the desired balance of properties that are useful forproviding cast precision optical articles. Moreover, many of thepolymeric compositions generally have too high a viscosity to be usefulfor optical casting purposes.

SUMMARY OF THE INVENTION

The present invention includes a curable composition comprising anoligomer having a plurality of pendent, ethylenically unsaturated,free-radically polymerizable functional groups, and having a T_(g)≧20°C. (preferably having a T_(g)≧50° C.); a free-radically polymerizablecrosslinking agent and/or a diluent monomer; and a photoinitiator. Thecomposition, when cured, is non-yellowing, exhibits low shrinkage andlow birefringence and low sensitivity to moisture, making it suitablefor many optical applications including, but not limited to opticallenses, optical fibers, prisms, diffractive lenses, microlenses,microlens arrays, Fresnel lenses, light guides, and optical films andcoatings. The composition is low viscosity so that it may be used as anoptical adhesive and in conventional molding operations, and buildmolecular weight by a chain-growth addition process. Further, articlesmay be prepared by cast and cure processes and thereby avoidbirefringence induced by injection molding processes.

Generally, curable systems containing a significant amount of solvent,monomers and reactive diluents can give rise to a significant increasein density when transformed from the uncured to the cured state causinga net shrinkage in volume. As is well known, shrinkage can causeunpredictable registration in precise molding operations such as thoserequired in manufacture of optical elements such as lenses. Shrinkagecan also create residual stress in such optical articles, which cansubsequently lead to optical defects, including high birefringence.

The present invention also provides shaped articles, including opticalarticles, and a method for preparing the same comprising, in oneembodiment, the steps of:

(1) mixing the components to form an optical casting composition,

(2) optionally degassing the composition,

(3) optionally heating the composition,

(4) introducing the composition into a suitable mold, and

(5) effecting polymerization, preferably photopolymerization, of thecomposition.

The present invention addresses the needs of the industry by providing arapid cure, solvent free, curable composition, to produce thickprecision optics such as optical lenses, light guides, prisms, etc.,with low birefringence for applications in electronic displays, cameras,binoculars, fax machines, bar code scanners, and optical communicationdevices. The present invention is especially useful in preparing prismssuch as those used in polarizing beam splitters (PBS's) used in opticalimager systems and optical reader systems. The term “optical imagersystem” as used herein is meant to include a wide variety of opticalsystems that produce an image for a viewer to view. Optical imagersystems of the present invention may be used, for example, in front andrear projection systems, projection displays, head-mounted displays,virtual viewers, heads-up displays, optical computing systems, opticalcorrelation systems, and other optical viewing and display systems.

A PBS is an optical component that splits incident light rays into afirst polarization component and a second polarization component.Traditional PBS's function based on the plane of incidence of the light,that is, a plane defined by the incident light ray and a normal to thepolarizing surface. The plane of incidence also is referred to as thereflection plane, defined by the reflected light ray and a normal to thereflecting surface. Based on the operation of traditional polarizers,light has been described as having two polarization components, a p andan s-component. The p-component corresponds to light polarized in theplane of incidence. The s-component corresponds to light polarizedperpendicular to the plane of incidence.

To achieve the maximum possible efficiency in an optical imaging system,a low f/# system is desirable (see, F. E. Doany et al., Projectiondisplay throughput; Efficiency of optical transmission and light-sourcecollection, IBM J. Res. Develop. V42, May/July 1998, pp. 387-398). Thef/# measures the light gathering ability of an optical lens and isdefined as:

f/#=f(focal length)÷D (diameter or clear aperture of the lens). The f/#(or F) measures the size of the cone of light that may be used toilluminate an optical element. The lower the f/#, the faster the lensand the larger the cone of light that may be used with that opticalelement. A larger cone of light generally translates to higher lightthroughput. Accordingly, a faster (lower f/#) illumination systemrequires a PBS able to accept light rays having a wider range ofincident angles. The maximum incident angle Θ_(max) (the outer rays ofthe cone of light) may be mathematically derived from the f/#;Θ_(max)=tan⁻¹((2F)⁻¹)

Traditional folded light path optical imaging systems have employed anoptical element know as a MacNeille polarizer. MacNeille polarizers takeadvantage of the fact that an angle exists, called Brewster's angle, atwhich no p-polarized light is reflected from an interface between twomedia of differing refractive index (n). Brewster's angle is given by:Θ_(B)=tan⁻¹(n ₁ /n ₀),

where n₀ is the refractive index of one medium, and n₁ is the refractiveindex of the other. When the angle of incidence of an incident light rayreaches the Brewster angle, the reflected beam portion is polarized inthe plane perpendicular to the plane of incidence. The transmitted beamportion becomes preferentially (but not completely) polarized in theplane parallel to the plane of incidence. In order to achieve efficientreflection of s-polarized light, a MacNeille polarizer is constructedfrom multiple layers of thin films of materials meeting the Brewsterangle condition for the desired angle. The film thicknesses are chosensuch that the film layer pairs form a quarter wave stack.

There is an advantage to this construction in that the Brewster anglecondition is not dependent on wavelength (except for dispersion in thematerials). However, MacNeille polarizers have difficulty achievingwide-angle performance due to the fact that the Brewster angle conditionfor a pair of materials is strictly met at only one angle of incidence.As the angle of incidence deviates from this angle a spectrallynon-uniform leak develops. This leak becomes especially severe as theangle of incidence on the film stack becomes more normal than theBrewster's angle. As will be explained below, there are also contrastdisadvantages for a folded light path projector associated with the useof p and s-polarization, referenced to the plane of reflection for eachray.

Typically, MacNeille PBS's are contained in glass cubes, wherein a PBSthin-film stack is applied along a diagonal plane of the cube. Bysuitably selecting the index of the glass in the cube, the PBS may beconstructed so that light incident normal to the face of the cube isincident at the Brewster angle of the PBS. However, the use of cubesgives rise to certain disadvantages, principally associated with thegeneration of thermal stress-induced birefringence that degrades thepolarization performance of the component. Even expensive pre-annealedcubes may suffer from this difficulty. Also cubes add significant weightto a compact system.

MacNeille-type PBS's reportedly have been developed capable ofdiscrimination between s- and p-polarized light at f/#'s as low asf/2.5, while providing extinction levels in excess of 100:1 betweenincident beams of pure s- or pure p-polarization. Unfortunately, asexplained below, when MacNeille-type PBS's are used in a folded lightpath with reflective imagers, the contrast is degraded due todepolarization of rays of light having a reflection plane rotatedrelative to the reflection plane of the principal ray. As used below,the term “depolarization” is meant to describe the deviation of thepolarization state of a light ray from that of the principal light ray.As light in a projection system generally is projected as a cone, mostof the rays of light are not perfectly parallel to the principal lightray. The depolarization increases as the f/# decreases, and is magnifiedin subsequent reflections from color selective films. This“depolarization cascade” has been calculated by some optical imagingsystem designers to effectively limit the f/# of MacNeille PBS basedprojectors to about 3.3, thereby limiting the light throughputefficiency of these systems. See, A. E. Rosenbluth et al., Contrastproperties of reflective liquid crystal light valves inprojectiondisplays, IBM J. Res. Develop. V42, May/July 1998, pp. 359-386.

As used herein:

“Actinic radiation” means photochemically active radiation and particlebeams. Actinic radiation includes, but is not limited to, acceleratedparticles, for example, electron beams; and electromagnetic radiation;for example, microwaves, infrared radiation, visible light, ultravioletlight, X-rays, and gamma-rays. The radiation can be monochromatic orpolychromatic, coherent or incoherent, and should be sufficientlyintense to generate substantial numbers of free radicals in the actinicradiation curable compositions used in the inventive compositions.

“(Meth)acryloyl groups” means both acryloyl and methacryloyl groups, andincludes acrylate, methacrylate, acrylamide and methacrylamide groups.

“Ethylenically unsaturated groups” include, but are not limited to,vinyl, (meth)acryloyl and the like.

“Melt processible” is used to refer to oligomer compositions thatpossess or achieve a suitable low viscosity for coating or molding attemperatures less than or equal to 100° C., using conventional moldingor coating equipment.

“Photocuring” and “photopolymerization” are used interchangeably in thisapplication to indicate an actinic radiation induced chemical reactionin which relatively simple molecules combine to form a chain or net-likemacromolecule.

“100% solids” means a composition free of unreactive species, such assolvents.

“Transmittance” of radiant energy refers to the passage of radiantenergy through a material.

“Transparency” may be considered as a degree of regular transmission,and thus the property of a material by which objects may be seen througha sheet thereof. A transparent material transmits light withoutsignificant diffusion or scattering.

BRIEF DESCRIPTION OF THE FIGURE

FIGS. 1 and 2 are schematics of a process of the invention.

DETAILED DESCRIPTION

The present invention provides curable materials comprising one or moreoligomers, preferably (meth)acryloyl oligomers having a plurality ofpendent, free-radically polymerizable functional groups, and having aT_(g)≧20° C. (preferably a T_(g)≧50° C.); a free-radically polymerizablecrosslinking agent and/or a diluent monomer, and a photoinitiator. Theoligomer may be selected from poly(meth)acrylate, polyurethane,polyepoxide, polyester, polyether, polysulfide, and polycarbonateoligomers.

In many embodiments, the present invention provides curable materialswith low shrinkage, residual stress and birefringence that is opticallyclear and non-yellowing for applications in precision optics andelectronic displays.

The composition of the present invention minimizes shrinkage andbirefringence due to optimum molecular weight of the oligomer andloading of the crosslinker and/or reactive diluent. The low shrinkagecompositions of this invention are particularly useful in moldingapplications or in any applications where accurate molding and/orregistration is required. The present invention provides newcompositions that may be formulated as 100% solids, cured byfree-radical means and that exhibit properties that meet or exceed thoseof the art. The present invention provides compositions that exhibitless than 5% shrinkage, and preferably less than 3%. The compositionsare low in viscosity and suitable for molding processes, includingprecision molding processes. The compositions generally have a viscosityless than 20,000 centipoise, less than 15,000 centipoise, or less than10,000 centipoise at application temperatures of 100° C. or less. Thecompositions generally have a viscosity of at least 100 centipoise, orat least 500 centipoise at application temperatures of 100° C. or less.

The articles of the invention may have a thickness greater than about0.5 millimeters, generally a birefringence (absolute) of less than1×10⁻⁶, light transmission greater than about 85%, preferably greaterthan 90%, and a CIELAB b* less than about 1.5 units, preferably lessthan about 1.0 unit for samples with thickness of 4.8 millimeters.

The composition generally comprises:

50 to 99 parts by weight, preferably 60 to 95 parts and most preferably70 to 95 parts of an oligomer having a plurality of pendentfree-radically polymerizable functional groups and having a Tg≧20° C.,preferably≧50° C.;

1 to 50 parts by weight, preferably 5 to 40 parts, and most preferably 5to 30 parts of a free-radically polymerizable crosslinking agent and/ora diluent monomer;

and 0.001 to 5 parts be weight, preferably 0. 001 to 1, most preferably0.01 to 0.1 parts of a photoinitiator, based on 100 parts by weight ofoligomer and crosslinking agent and/or reactive diluent monomer.

In some preferred embodiments, the crosslinking agent comprises 1 to 40parts by weight, preferably 1 to 30 parts by weight, and most preferably1 to 20 parts by weight. In some embodiments, the reactive diluentcomprises less than 25 parts by weight, preferably less than 15 parts byweight and most preferably less than 10 parts by weight.

The oligomer may be selected from poly(meth)acrylate, polyurethane,polyepoxide, polyester, polyether, polysulfide, and polycarbonateoligomers. Suitable oligomers for compositions of the invention have aglass transition temperature (Tg) of greater than about 20° C.,preferably greater than about 50° C. The (meth)acryloyl-functionalizedoligomers useful in the compositions can be represented by the generalstructure (I) below:where R¹ is H or CH₃;   (I)Z isR² is (CH₂)_(m), where m is 1 to 6;

X is an polyvalent radical group resulting from oligomerized monomerunits (i.e. the oligomer chain), such as a poly(meth)acryloyl,polyurethane, polyepoxide, polyester, polyether, polysulfide,polyepoxide, and polycarbonate; and n is greater than or equal to 2. Theoligomer having a plurality of pendent or terminal, free-radicallypolymerizable groups is selected such that free-radicalhomopolymerization of the oligomer (e.g., by photo- or thermalinitiation) results in a polymer having a glass transition temperatureat or above 20° C, preferably at or above 50° C.

For structure (I), wherein n is greater than or equal to 2, an oligomerhaving a plurality of pendent or terminal (meth)acryloyl groups isprovided. In certain embodiments of the invention, oligomers areutilized having n such that the degree of polymerization from 10 to 300,preferably 15 to 200, more preferably 20 to 200. Preferably Formula Idefines a (meth)acrylated aliphatic urethane oligomer, (meth)acrylatedaliphatic polyester oligomer, (meth)acrylated aliphatic epoxy oligomer,(meth)acrylated aliphatic polycarbonate oligomer, or a (meth)acrylatedaliphatic acrylic oligomer. Methods for making such oligomers are wellknown in the art, and many useful free-radically polymerizable oligomersare commercially available.

(Meth)acrylated urethanes are multifunctional (meth)acrylate esters ofhydroxy terminated isocyanate extended polyols, polyesters orpolyethers. (Meth)acrylated urethane oligomers can be synthesized, forexample, by reacting a diisocyanate or other polyvalent isocyanatecompound with a polyvalent radical polyol (including polyether andpolyester polyols) to yield an isocyanate terminated urethaneprepolymer. A polyester polyol can be formed by reacting a polybasicacid (e.g., terephthalic acid or maleic acid) with a polyhydric alcohol(e.g., ethylene glycol or 1,6-hexanediol). A polyether polyol useful formaking the acrylate functionalized urethane oligomer can be chosen from,for example, polyethylene glycol, polypropylene glycol,poly(tetrahydrofuran), poly(2-methyl-tetrahydrofuran),poly(3-methyl-tetrahydrofuran) and the like. Alternatively, the polyollinkage of an acrylated urethane oligomer (structure (II)) can be apolycarbonate polyol.

Subsequently, (meth)acrylates having a hydroxyl group can then bereacted with the terminal isocyanate groups of the prepolymer. Botharomatic and the preferred aliphatic isocyanates can be used to reactwith the urethane to obtain the oligomer. Examples of diisocyanatesuseful for making the acrylated oligomers are 2,4-tolylene diisocyanate,2,6-tolylene diiscyanate, 1,3-xylylene diisocyanate, 1,4-xylylenediisocyanate, 1,6-hexane diisocyanate, isophorone diisocyanate and thelike. Examples of hydroxy terminated acrylates useful for making theacrylated oligomers include, but are not limited to,2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate,4-hydroxybutyl acrylate, polyethylene glycol(meth)acrylate and the like.

A (meth)acrylated urethane oligomer can be, for example, any urethaneoligomer having at least two acrylate functionalities and generally lessthan about six functionalities. Suitable (meth)acrylated urethaneoligomers are also commercially available such as, for example, thoseknown by the trade designations PHOTOMER 6008, 6019, 6184 (aliphaticurethane triacrylates) available from Henkel Corp.; EBECRYL 220(hexafunctional aromatic urethane acrylate of 1000 molecular weight),EBECRYL 284 (aliphatic urethane diacrylate of 1200 molecular weightdiluted with 12% of 1,6-hexanediol diacrylate), EBECRYL 4830 (aliphaticurethane diacrylate of 1200 molecular weight diluted with 10% of tetraethylene glycol diacrylate), and EBECRYL 6602 (trifunctional aromaticurethane acrylate of 1300 molecular weight diluted with 40% oftrimethylolpropane ethoxy triacrylate), available from UCB Chemical; andSARTOMER CN1963, 963E75, 945A60, 963B80, 968, and 983) available fromSartomer Co., Exton, Pa.

Alternatively, the acrylate functionalized oligomers can be polyesteracrylate oligomers, acrylated acrylic oligomers, polycarbonate acrylateoligomers or polyether acrylate oligomers. Suitable acrylated acrylicoligomers include, for example, commercially available products such asEbecryl 745 and 1710 both of which are available from UCB Chemicals(Smyrna, Ga.). Useful polyester acrylate oligomers include CN293, CN294,and CN2250, 2281, 2900 from Sartomer Co. (Exton, Pa.) and EBECRYL 80,657, 830, and 1810 from UCB Chemicals (Smyrna, Ga.). Suitable polyetheracrylate oligomers include CN501, 502, and 551 from Sartomer Co. (Exton,Pa.). Useful polycarbonate acrylate oligomers can be prepared accordingto US 6451958 (Sartomer Technology Company Inc., Wilmington, Del.).

(Meth)acrylated epoxies are multifunctional (meth)acrylate esters ofepoxy resins, such as the (meth)acrylated esters of bisphenol-A epoxyresin. Examples of commercially available acrylated epoxies includethose known by the trade designations EBECRYL 600 (bisphenol A epoxydiacrylate of 525 molecular weight), EBECRYL 605 (EBECRYL 600 with 25%tripropylene glycol diacrylate), EBECRYL 3700 (bispenol A diacrylate of524 molecular weight) and EBECRYL 3720H (bisphenol A diacrylate of 524molecular weight with 20% hexanediol diacrylate) available from UCBChemical, Smyrna, Ga.; and PHOTOMER 3016 (bisphenol A epoxy acrylate),PHOTOMER 3016-40R (epoxy acrylate and 40% tripropylene glycol diacrylateblend), and PHOTOMER 3072 (modified bisphenol A acrylate, etc.)available from Henkel Corp., Hoboken, N.J.

In a preferred embodiment, the oligomer generally comprises polymerizedacryloyl monomer units comprising:

-   -   a) 50 to 99 parts by weight, preferably 60 to 97 parts by        weight, most preferably 80 to 95 parts by weight of        (meth)acryloyl monomer units homopolymerizable to a polymer        having a glass transition temperature≧20° C., preferably≧50° C.,        preferably the (meth)acryloyl monomer units are (meth)acrylate        monomer units    -   b) 1 to 50 parts by weight, preferably 3 to 40 parts by weight,        most preferably 5 to 20 parts by weight, of monomer units having        a pendent, free-radically polymerizable functional group,    -   c) less than 40 parts by weight, preferably less than 30 parts        by weight, most preferably less than 20 parts by weight, of        monomer units homopolymerizable to a polymer having a glass        transition temperature less than 20° C., based on 100 parts by        weight of a) and b).

The first component oligomer comprises one or more high T_(g) monomers,which if homopolymerized, yield a polymer having a T_(g)>20° C.,preferably>50° C. Preferred high T_(g) monomers are monofunctional(meth)acrylate esters of mono- and bicyclic aliphatic alcohols having atleast 6 carbon atoms, and of aromatic alcohols. Both the cycloaliphaticand aromatic groups may be substituted, for example, by C₁₋₆ alkyl,halogen, sulfur, cyano, and the like. Especially preferred high T_(g)monomers include 3,5-dimethyladamantyl(meth)acrylate;isobornyl(meth)acrylate; 4-biphenyl(meth)acrylate; phenyl(meth)acrylate;benzyl methacrylate; and 2-naphthyl(meth)acrylate;dicyclopentadienyl(meth)acrylate. Mixtures of high T_(g) monomers mayalso be used. Providing the monomer can be polymerized with the rest ofthe monomers that comprise the (meth)acrylate monomers, any high T_(g)monomer including styrene, vinylesters and the like, can be used.However, the high T_(g) monomer is typically an acrylate or methacrylateester.

Other high T_(g) monomers include C₁-C₂₀ alkyl (meth)acrylates such asmethyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butylmethacrylate, isobutyl methacrylate, t-butyl(meth)acrylate, stearylmethacrylate, cyclohexyl methacrylate, 3,3,5-trimethylcyclohexylmethacrylate, tetrahydrofurfuryl methacrylate, allyl methaylacrylate,bromoethyl methacrylate; styrene; vinyl toluene; vinyl esters such asvinyl propionate, vinyl acetate, vinyl pivalate, and vinyl neononanoate;acrylamides such as N,N-dimethyl acrylamide, N,N-diethyl acrylamide,N-isopropyl acrylamide, N-octyl acrylamide, and t-butyl acrylamide, and(meth)acrylonitrile. Blends of high T_(g) monomers may be used.

Most preferred high T_(g) monomers are selected from linear, branched,cyclo, bridged cyclo aliphatic(meth)acrylates, such asisobornyl(meth)acrylate, cyclohexyl methacrylate,3,3,5-trimethylcyclohexyl methacrylate, methyl methacrylate, ethylmethacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutylmethacrylate, t-butyl(meth)acrylate, stearyl methacrylate, and mixturesthere of, for their environmental (heat and light) stability.

The first component oligomer of the composition comprises one or morependent groups that include free-radically polymerizable unsaturation.Preferred pendent unsaturated groups include (meth)acryloyl,(meth)acryloxy, propargyl, and (meth)acrylamido. Such pendent groups canbe incorporated into the polymer in at least two ways. The most directmethod is to include among the monomer units of ethylenedi(meth)acrylate, 1,6-hexanediol diacrylate (HDDA), or bisphenol-Adi(meth)acrylate. Useful polyunsaturated monomers include allyl,propargyl, and crotyl(meth)acrylates, trimethylolpropane triacrylate,pentaerythritol triacrylate, and allyl 2-acrylamido-2,2-dimethylacetate.

Using the “direct method” of incorporating the pendent, free-radicallypolymerizable functional group, useful functional monomers include thoseunsaturated aliphatic, cycloaliphatic, and aromatic compounds having upto about 36 carbon atoms that include a functional group capable of freeradical addition such as those groups containing a carbon-carbon doublebond including vinyl, vinyloxy, (meth)acrylate, (meth)acrylamido, andacetylenic functional groups.

Examples of polyethylenically unsaturated monomers that can be usedinclude, but are not limited to, polyacrylic-functional monomers such asethylene glycol diacrylate, propylene glycol dimethacrylate,trimethylolpropane triacrylate, 1,6-hexamethylenedioldiacrylate,pentaerythritol di-, tri-, and tetraacrylate, and1,12-dodecanedioldiacrylate; olefinic-acrylic-functional monomers suchas allyl methacrylate, 2-allyloxycarbonylamidoethyl methacrylate, and2-allylaminoethyl acrylate; allyl 2-acrylamido-2,2-dimethylacetate;divinylbenzene; vinyloxy group-substituted functional monomers such as2-(ethenyloxy)ethyl(meth)acrylate, 3-(ethynyloxy)-1-propene,4-(ethynyloxy)-1-butene, and4-(ethenyloxy)butyl-2-acrylamido-2,2-dimethylacetate, and the like.Useful polyunsaturated monomers, and useful reactive/co-reactivecompounds that may be used to prepare a polymer having pendentunsaturation are described in greater detail in U.S. Pat. No. 5,741,543(Winslow et al.).

Preferred polyunsaturated monomers are those where the unsaturatedgroups are of unequal reactivity. Those skilled in the art recognizethat the particular moieties attached to the unsaturated groups affectthe relative reactivities of those unsaturated groups. For example,where a polyunsaturated monomer having unsaturated groups of equalreactivity (e.g., HDDA) is used, premature gellation of the compositionmust be guarded against by, for example, the presence of oxygen, whichacts as a radical scavenger. Conversely, where a polyunsaturated monomerhaving unsaturated groups of differing reactivities is used, the morereactive group (such as (meth)acrylate as (meth)acrylamido)preferentially is incorporated into the polymer backbone before the lessreactive unsaturated group (such as vinyl, allyl, vinyloxy, oracetylenic) reacts to crosslink the composition. The direct method isgenerally not preferred due to difficulty in control of branching andpremature gellation.

An indirect, but preferred, method of incorporating pendent groups thatcomprise polymerizable unsaturation into the first polymer is to includeamong the monomer units of the polymer some that comprise a reactivefunctional group. Useful reactive functional groups include, but are notlimited to, hydroxyl, amino (especially secondary amino), oxazolonyl,oxazolinyl, acetoacetyl, carboxyl, isocyanato, epoxy, aziridinyl, acylhalide, and cyclic anhydride groups. Preferred among these are carboxyl,hydroxyl and aziridinyl groups. These pendent reactive functional groupsare reacted with unsaturated compounds that comprise functional groupsthat are co-reactive with the reactive pendent functional group. Whenthe two functional groups react, an oligomer with pendent unsaturationresults.

Using the “indirect method” of incorporating the pendent, free-radicallypolymerizable functional groups, useful reactive functional groupsinclude hydroxyl, secondary amino, oxazolinyl, oxazolonyl, acetyl,acetonyl, carboxyl, isocyanato, epoxy, aziridinyl, acyl halide,vinyloxy, and cyclic anhydride groups. Where the pendent reactivefunctional group is an isocyanato functional group, the co-reactivefunctional group preferably comprises a secondary amino or hydroxylgroup. Where the pendent reactive functional group comprises a hydroxylgroup, the co-reactive functional group preferably comprises a carboxyl,isocyanato, epoxy, anhydride, or oxazolinyl group. Where the pendentreactive functional group comprises a carboxyl group, the co-reactivefunctional group preferably comprises a hydroxyl, amino, epoxy,isocyanate, or oxazolinyl group. Most generally, the reaction is betweennucleophilic and electrophilic functional groups that react by adisplacement or condensation mechanism.

Representative examples of useful co-reactive compounds includehydroxyalkyl (meth)acrylates such as 2-hydroxyethyl(meth)acrylate,4-hydroxybutyl(meth)acrylate, and2-(2-hydroxyethoxy)ethyl(meth)acrylate; aminoalkyl(meth)acrylates suchas 3-aminopropyl(meth)acrylate and 4-aminostyrene; oxazolinyl compoundssuch as 2-ethenyl-1,3-oxazolin-5-one and2-propenyl-4,4-dimethyl-1,3-oxazolin-5-one; carboxy-substitutedcompounds such as (meth)acrylic acid and 4-carboxybenzyl (meth)acrylate;isocyanato-substituted compounds such as isocyanatoethyl(meth)acrylateand 4-isocyanatocyclohexyl(meth)acrylate; epoxy-substituted compoundssuch as glycidyl(meth)acrylate; aziridinyl-substituted compounds such asN-acryloylaziridine and 1-(2-propenyl)-aziridine; and acryloyl halidessuch as (meth)acryloyl chloride.

Preferred functional monomers have the general formulawherein R¹ is hydrogen, a C₁ to C₄ alkyl group, or a phenyl group,preferably hydrogen or a methyl group; R² is a single bond or a divalentlinking group that joins an ethylenically unsaturated group topolymerizable or reactive functional group A and preferably contains upto 34, preferably up to 18, more preferably up to 10, carbon and,optionally, oxygen and nitrogen atoms and, when R²is not a single bond,is preferably selected from

wherein R³ is an alkylene group having 1 to 6 carbon atoms, a 5- or6-membered cycloalkylene group having 5 to 10 carbon atoms, or analkylene-oxyalkylene in which each alkylene includes 1 to 6 carbon atomsor is a divalent aromatic group having 6 to 16 carbon atoms; and A is afunctional group, capable of free-radical addition to carbon-carbondouble bonds, or a reactive functional group capable of reacting with aco-reactive functional group for the incorporation of a free-radicallypolymerizable functional group.

It will be understood, in the context of the above description of thefirst component oligomer, that the ethylenically-unsaturated monomerpossessing a free-radically polymerizable group is chosen such that itis free-radically polymerizable with the crosslinking agent and reactivediluent. The reactions between functional groups provide a crosslink byforming a covalent bond by free-radical addition reactions ofethylenically-unstaurated groups between components. In the presentinvention the pendent functional groups react by an addition reaction inwhich no by-product molecules are created, and the exemplified reactionpartners react by this preferred mode.

Where the curable composition is to be processed using high temperaturesand the direct method of including pendent unsaturation has been used,care must be taken not to activate those pendent groups and causepremature gelation. For example, hot-melt processing temperatures can bekept relatively low and polymerization inhibitors can be added to themixture. Accordingly, where heat is to be used to process thecomposition, the above-described indirect method is the preferred way ofincorporating the pendent unsaturated groups.

The oligomer may optionally further comprise lower T_(g)alkyl(meth)acrylate esters or amides that may be homopolymerized topolymers having a T_(g) of less than 20° C. Alkyl(meth)acrylate estermonomers useful in the invention include straight-chain, cyclic, andbranched-chain isomers of alkyl esters containing C₁-C₂₀ alkyl groups.Due to T_(g) and side chain crystallinity considerations, preferredlower T_(g) alkyl(meth)acrylate esters are those having from C₁-C₈ alkylgroups. Useful specific examples of alkyl(meth)acrylate esters include:methyl acrylate, ethyl acrylate, n-propyl acrylate, butyl acrylate,iso-amyl (meth)acrylate, n-hexyl(meth)acrylate, n-heptyl(meth)acrylate,n-octyl(meth)acrylate, iso-octyl(meth)acrylate,2-ethylhexyl(meth)acrylate, iso-nonyl(meth)acrylate, anddecyl(meth)acrylate. Most preferred (meth)acrylate esters include methylacrylate, ethyl acrylate, butyl acrylate, isooctyl(meth)acrylate,2-ethylhexyl(meth)acrylate, cyclohexyl acrylate. The lower T_(g)alkyl(meth)acrylate esters are added in such an amount such that theresulting oligomer has a T_(g) of 20° C. or greater. In general, suchlow Tg monomers are used in amounts of 40 parts by weight or less,preferably 30 parts by weight or less, most preferable 20 parts byweight or less.

The theoretical T_(g) of an oligomer may be calculated, for example,using the Fox equation, 1/T_(g)=(w¹/T_(g) ¹+w²/T_(g) ²), where w¹ and w²refer to the weight fraction of the two components and T_(g) ¹ and T_(g)² refer to the glass transition temperature of the two components, asdescribed for example in L. H. Sperling, “Introduction to PhysicalPolymer Science”, 2^(nd) Edition, John Wiley & Sons, New York, p. 357(1992) and T. G. Fox, Bull. Am. Phys. Soc., 1, 123 (1956), which areincorporated herein by reference. Using the T_(g) of the componentmonomers, and an estimate of the weight fractions thereof in theoligomer, one may calculate the T_(g) of the resulting oligomer. Asunderstood by one skilled in the art, the Fox equation may be used for asystem with more than two components.

The oligomer may be prepared using radical polymerization techniques bycombining an initiator and monomers in the presence of a chain transferagent. In this reaction, a chain transfer agent transfers the activesite on one growing chain to another molecule that can then start a newchain so the degree of polymerization may be controlled. The degree ofpolymerization of the resulting oligomer may be 10 to 300, preferably 15to 200, more preferably 20 to 200. It has been found if the degree ofpolymerization is too high, the composition is too high in viscosity,and not easily melt processible, and. Conversely, if the degree ofpolymerization is too low, the shrinkage of the cured composition isexcessive and leads to high birefringence in the cured composition.

Chain transfer agents may be used when polymerizing the monomersdescribed herein to control the molecular weight of the resultingoligomer. Suitable chain transfer agents include halogenatedhydrocarbons (e.g., carbon tetrabromide) and sulfur compounds (e.g.,lauryl mercaptan, butyl mercaptan, ethanethiol, and 2-mercaptoethylether, isooctyl thioglycolate, t-dodecylmercaptan,3-mercapto-1,2-propanediol). The amount of chain transfer agent that isuseful depends upon the desired molecular weight of the oligomer and thetype of chain transfer agent. The chain transfer agent is typically usedin amounts from about 0.1 parts to about 10 parts; preferably 0.1 toabout 8 parts; and more preferably from about 0.5 parts to about 6 partsbased on total weight of the monomers.

Suitable initiators for this oligomerization reaction include, forexample, thermal and photo initiators. Useful thermal initiators includeazo compounds and peroxides. Examples of useful azo compounds include2,2′-azobis(2,4-dimethylpentanenitrile), (Vazo 52, commerciallyavailable from E. I. duPont de Nemours & Co.);2,2′-azobis(isobutyronitrile), (Vazo 64, commercially available from E.I. duPont de Nemours & Co.); 2,2′-azobis(2-methylbutyronitrile), (Vazo67, commercially available from E. I. duPont de Nemours & Co.);1,1′-azobis(cyanocyclohexane), (Vazo 88, commercially available from E.I. duPont de Nemours & Co.); 1,1′-azobis(1-cyclohexane-1-carbonitrile),(V-40, commercially available from Wako Pure Chemical Industries, Ltd.);and dimethyl 2,2′-azobis(isobutyrate), (V-601, commercially availablefrom Wako Pure Chemical Industries, Ltd.). Examples of useful peroxidesinclude benzoyl peroxide; di-t-amyl peroxide, t-butyl peroxy benzoate,2,5-dimethyl-2,5Di-(t-butylperoxy)hexane,2,5-dimethyl-2,5-Di-(t-butylperoxy)hexyne-3, lauroyl peroxide, andt-butyl peroxy pivalate. Useful organic hydroperoxides include but arenot limited to compounds such as t-amyl hydroperoxide and t-butylhydroperoxide.

Useful photoinitiators include benzoin ethers such as benzoin methylether and benzoin butyl ether; acetophenone derivatives such as2,2-dimethoxy-2-phenyl-acetophenone and 2,2-diethoxy acetophenone; andacylphosphine oxide derivatives and acylphosphonate derivatives such asdiphenyl-2,4,6-trimethylbenzoylphosphine oxide,isopropoxy(phenyl)-2,4,6-trimethylbenzoylphosphine oxide, and dimethylpivaloylphosphonate. Of these, 2,2-dimethoxy-2-phenyl-acetophenone ispreferred. The initiator is typically used at a level of 0.001 to 5parts by weight per 100 parts by weight monomer(s).

The composition further comprises a crosslinking agent having aplurality of pendent, ethylenically unsaturated, free-radicallypolymerizable functional groups. Useful crosslinking agents have anaverage functionality (average number of ethylenically unsaturated,free-radically polymerizable functional groups per molecule) of greaterthan one, and preferably greater than or equal to two. The functionalgroups are chosen to be copolymerizable with the pendent ethylenicallyunsaturated, free-radically polymerizable functional groups on the firstcomponent oligomer. Useful functional groups include those described forthe first component oligomer and include, but are not limited to vinyl,vinyloxy, (meth)acryloyl and acetylenic functional groups.

Useful crosslinking agents have the general formula:R-(Z)_(n)

where Z is a free-radically polymerizable functional group such as acarbon-carbon double bond, n is greater than 1 and R is an organicradical having a valency of n. Preferably R is an aliphatic alkylradical of valency n which may be linear or branched.

Examples of such crosslinking agents include: C₂-C₁₈ alkylenedioldi(meth)acrylates, C₃-C₁₈ alkylenetriol tri(meth)acrylates, such as1,6-hexanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate,propoxylated trimethylolpropane triacrylate such as CD501 from SratomerCo., Exton, Pa., triethyleneglycol di(meth)acrylate, pentaeritritoltri(meth)acrylate, and tripropyleneglycol di(meth)acrylate, anddi-trimethylolpropane tetraacrylate, polyalkyleneglycol dimethacrylatesuch as Bisomer™ EP 100DMA from Cognis Co. For ease of mixing, thepreferred crosslinking agent is not a solid material at applicationtemperatures.

The composition according to the invention may comprise at least onereactive diluent. The reactive diluents can be used to adjust theviscosity of the composition. Thus, the reactive diluents can each be alow viscosity monomer containing at least one functional group capableof polymerization when exposed to actinic radiation. For example, vinylreactive diluents and (meth)acrylate monomer diluents may be used.

The functional group present on the reactive diluents may be the same asthat used in the curable (meth)acrylate oligomer. Preferably, theradiation-curable functional group present in the reactive diluent iscapable of copolymerizing with the radiation-curable functional grouppresent on the radiation-curable oligomer. The reactive diluentsgenerally have a molecular weight of not more than about 550 or aviscosity at room temperature of less than about 500 mPascal.sec(measured as 100% diluent).

The reactive diluent may comprise monomers having a (meth)acryloyl orvinyl functionality and a C₁-C₂₀ alkyl moiety. Examples of such reactivediluents are ethyl (meth)acrylate, isopropyl (meth)acrylate,t-butyl(meth)acrylate, n-butyl(meth)acrylate, cyclohexyl(meth)acrylate,isobornyl(meth)acrylate, isooctyl(meth)acrylate,2-ethylhexyl(meth)acrylate, stearyl(meth)acrylate,phenoxyethyl(meth)acrylate, benzyl(meth)acrylate and the like. Lowvolatile alkyl(meth)acrylates such as isobornyl(meth)acrylate,2-ethylhexyl(meth)acrylate, cyclohexyl(meth)acrylate,isooctyl(meth)acrylate, stearyl(meth)acrylate,phenoxyethyl(meth)acrylate, benzyl(meth)acrylate are preferred reactivediluents.

The reactive diluent is preferably added in such an amount that theshrinkage of the cured compositions does not exceed around 5%,preferably not above around 3%, as measured by the test method describedherein. Suitable amounts of the reactive diluents have been found to beless than about 25 parts by weight, preferably about 0 to about 15 partsby weight, and more preferably about 0 to about 10 parts by weight.Preferably, the sum of the amounts of the reactive diluent and thecrosslinking agent is less than 40 parts by weight.

The components of the composition may be combined and cured with aphotoinitiator. The photoinitiator improves the rate of cure and percentconversion of the curable compositions, but the depth of cure (ofthicker coatings or shaped articles) may be deleteriously affected asthe photoinitiator may attenuate the transmitted light that penetratesthe thickness of the sample. The photoinitiator is used in an amount ofless than 1.0 weight %, preferably less than 0.1 weight %, mostpreferably less than 0.05 weight %.

Conventional photoinitiators can be used. Examples includebenzophenones, acetophenone derivatives, such asα-hydroxyalkylphenylketones, benzoin alkyl ethers and benzil ketals,monoacylphosphine oxides, and bis-acylphosphine oxides. Preferredphotoinitiators are ethyl 2,4,6-trimethylbenzoylphenyl phosphinate(Lucirin TPO-L) available from BASF, Mt. Olive, N.J.,2-hydroxy-2-methyl-1-phenyl-propan-1-one (IRGACURE ₁₁₇₃™, CibaSpecialties), 2,2-dimethoxy-2-phenyl acetophenone (IRGACURE ₆₅₁™, CibaSpecialties), phenyl bis(2,4,6-trimethyl benzoyl)phosphine oxide(IRGACURE 819, Ciba Specialties). Other suitable photoinitiators includemercaptobenzothiazoles, mercaptobenzooxazoles and hexaryl bisimidazole.Often, mixtures of photoinitiators provide a suitable balance ofproperties.

The compositions can then be applied to the desired substrate or addedto a mold and exposed to actinic radiation such as UV light. Thecomposition may be exposed to any form of actinic radiation, such asvisible light or UV radiation, but is preferably exposed to UVA (320 to390 nm) or UVB (395 to 445 nm) radiation. Generally, the amount ofactinic radiation should be sufficient to form a non-tacky,dimensionally stable solid mass. Generally, the amount of energyrequired for curing the compositions of the invention ranges from about0.2 to 20.0 J/cm².

The photopolymerization may be effected by any suitable light sourceincluding carbon arc lights, low, medium, or high pressure mercury vaporlamps, swirl-flow plasma arc lamps, xenon flash lamps, ultraviolet lightemitting diodes, and ultraviolet light emitting lasers For manyapplications it may be desirable to use an LED light source or array toeffect the curing. Such LED sources may effect a faster cure and provideless heat to the composition during cure. One suitable LED source is theNorlux large area array, series 808 (available from Norlux, CarolStream, Ill.).

A preferred method of making the oligomer is through an adiabaticpolymerization method (see for example, U.S. Pat. No. 5,986,011 (Ellis)or U.S. Pat. No. 5,753,768 (Ellis)), incorporated herein by reference.In such a polymerization, the polymerization initiator(s) may be used ata low level, to reduce color due to the initiator fragments incorporatedinto the polymer. Further, during an adiabatic polymerization, partlybecause of low initiator levels, and partly due to the increasingtemperature profile that accompanies polymerization, conditions can beselected such that the initiator is essentially completely consumedduring the polymerization or at the end of the polymerization. Havingall thermal polymerization initiator consumed advantageously prevents orreduces unwanted polymerization and crosslinking during thefunctionalization step of the oligomer using the “indirect method” ofincorporating the pendent, free-radically polymerizable functionalgroups (described herein). Further, having no significant traces ofthermal initiator present beneficially improve the stability of thefunctionalized oligomer during storage and transport, prior to moldingand further curing.

The adiabatic polymerization process comprises the steps of:

-   (a) providing the oligomer composition of the invention in a batch    reactor;-   (b) deoxygenating the mixture, wherein step (b) can optionally at    least partially overlap with step (c);-   (c) heating the mixture to a sufficient temperature to generate    sufficient initiator free radicals from at least one thermal free    radical initiator so as to initiate polymerization;-   (d) allowing the mixture to polymerize under essentially adiabatic    conditions to yield an at least partially polymerized mixture;-   (e) optionally heating the mixture to generate free radicals from    some or all of any initiator that has not generated initiator free    radicals, followed by allowing the mixture to polymerize under    essentially adiabatic conditions to yield a further polymerized    mixture; and-   (f) optionally repeating step (e) one or more times.-   (g) optionally repeating steps (a) to (e) one or more times with    cooling between repeats.    Step (g) is useful if the monomers have a heat of reaction such that    it is difficult to achieve the desired conversion to oligomer in one    adiabatic polymerization step without getting too hot. Multiple    repeats of steps (a) to (e) with cooling between repeats to the    proper temperature(s) and then polymerizing adiabatically further in    the one or more repeats can be beneficial to control the final    polymerization temperature to a desired level. This may prevent    yellowing due to polymer degradation as result of the heat of    polymerization.

By appropriately selecting initiator(s) and amounts in step (a) andoptional use of step (g), the conversion to polymer can beadvantageously controlled to be sufficiently high so as to providecurable materials with low shrinkage, residual stress and birefringence.Further, in some instances, the functionalization and addition ofreactive diluents can then be performed while in the same reactionequipment minimizing contamination and oxidation of the final curableformulation.

The composition and process for making optical products of the presentinvention are applicable to a variety of applications needing opticalelements including, for example, optical lenses such as Fresnel lenses,prisms, optical films, such as high index of refraction films,non-warping and low birefringence film e.g., microreplicated films suchas totally internal reflecting films, or brightness enhancing films,flat films, multilayer films, retroreflective sheeting, optical lightfibers or tubes, and others. Such optical products are useful in opticalassemblies, optical projection systems, such as projection televisions,as well as displays and other devices containing the optical assemblies.The optical products of this invention include articles that arecurrently prepared from ground glass, or injection molded plastic.

Such articles may have a thickness of about 0.5 mm or greater, and canbe prepared from a curable composition of this invention which is madeby mixing in a suitable vessel, in any convenient order, the oligomer,crosslinking agent and/or reactive diluent, and a photoinitiator. Mixingis continued until the components of the composition are in a singlephase. Thicknesses of 25 mm articles have been achieved using thecomposition and curing process of this invention.

At the time of use, the composition is preferably degassed using avacuum of less than about 25 Torr or by flowing the composition in athin film past a suitable boundary. The degassed composition isintroduced, optionally using a pressure of about 2 to 400 Kg/cm², into amold corresponding to the shape of the article that is desired to beprepared. Such molds are generally made of plastic, glass or metal, orcombinations thereof.

In one embodiment, the curable composition may be applied to the surfaceof the mold having the requisite shape or to mold elements correspondingto the desired optical article, such as a lens. The volume of curablecomposition that enters the mold or mold elements can be controlled bysliding a squeegee across the surface of the mold. The amount of curablecomposition can also be applied by other known coating techniques, suchas by the use of a roller. If desired, heating may be used to reduce theviscosity of the composition and provide more efficient molding. Asdescribed, many embodiments of the invention are melt-processible, i.e.possess or achieve a suitable low viscosity for coating or molding attemperatures less than or equal to 100° C.

The mold elements may be completely filled or may be partially filled.If the photopolymerizable composition is a 100% solids, non-shrinking,curable material, then the shape of the cured composition will remainthe same as that of the mold elements. However, if thephotopolymerizable composition shrinks as it hardens, then the liquidwill contract, creating unreliable registration, and introducing opticaldefects. Preferably, the photopolymerizable composition includesmaterials that shrink by less than about 5% by volume, and preferablyless than about 3%, during curing.

To initiate photopolymerization, the molds are filled, placed under asource of actinic radiation such as a high-energy ultraviolet sourcehaving a duration and intensity of such exposure to provide foressentially complete (greater than 80%) polymerization of thecomposition contained in the molds. If desired, filters may be employedto exclude wavelengths that may deleteriously affect the reactivecomponents or the photopolymerization. Photopolymerization may beeffected via an exposed surface of the curable composition, or“through-mold” by appropriate selection of a mold material having therequisite transmission at the wavelengths necessary to effectpolymerization.

Photoinitiation energy sources emit actinic radiation, i.e., radiationhaving a wavelength of 700 nanometers or less which is capable ofproducing, either directly or indirectly, free radicals capable ofinitiating addition polymerization and chain-growth polymerization ofthe optical casting resins of this invention. Preferred photoinitiationenergy sources emit ultraviolet radiation, i.e., radiation having awavelength between about 180 and 460 nanometers, includingphotoinitiation energy sources such as carbon arc lights, low, medium,or high pressure mercury vapor lamps, swirl-flow plasma arc lamps, xenonflash lamps, ultraviolet light emitting diodes, and ultraviolet lightemitting lasers. Particularly preferred ultraviolet light sources arexenon flash lamps available from Xenon Corp, Wilburn, Mass., such asmodels RC-600, RC-700 and RC-747 pulsed UV-V is curing systems and LEDsources such as Norlux Series 808 large area array, (available fromNorlux, Carol Stream, Ill.). Although not preferred, the curablecomposition may also use convention thermal initiators, previouslydescribed.

In some embodiments, the optical product can contain one or morefeatures, such as flat or curved surfaces (including convex and concavesurfaces), or replicated or microreplicated surfaces (such as Fresnellenses), either of which can be derived from the composition of theinvention and a suitable mold. Structure-bearing articles can beconstructed in a variety of forms, including those including pluralityof linear prismatic structures having a series of alternating tips andgrooves. An example of such a film is BEF, having regular repeatingpattern of symmetrical tips and grooves. Other examples include patternsin which the tips and grooves are not symmetrical and in which the size,orientation, or distance between the tips and grooves is not uniform.Several examples of surface structure bearing articles useful asbrightness enhancing films are described in U.S. Pat. No. 5,175,030, (Luet al.) and U.S. Pat. No. 5,183,597, (Lu) said descriptions beingincorporated herein by reference.

According to the descriptions of Lu and Lu et al., a structure-bearingoptical product can be prepared by a method including the steps of (a)preparing a polymerizable composition; (b) depositing the polymerizablecomposition onto a master negative microstructured molding surface in anamount barely sufficient to fill the cavities of the master; (c) fillingthe cavities by moving a bead of the polymerizable composition between apreformed base and the master, at least one of which is flexible; and(d) curing the composition. The master can be metallic, such as nickel,nickel-plated copper or brass, or can be a thermoplastic material thatis stable under the polymerization conditions, and that preferably has asurface energy that allows clean removal of the polymerized materialfrom the master.

In a preferred embodiment, the optical article comprises a polarizingbeam splitter, wherein incident light is split into first and secondsubstantially polarized beam states that may be used in an image displaysystem. As shown in FIG. 1, the beam splitter comprises a first prism(60 a), a second prism (60 b) and a polarizing layer having a pass axisdisposed therebetween (20). At least one prism comprises the instantcured composition. Each of the prisms has a first surface coincidentwith the polarizing layer, and two or more outer surfaces. As usedherein, the term “prism” refers to an optical element that controls theangular transmission of incident light through the polarizing layer, andthe angular character of light exiting the article. The prisms may beregular polygons, such as triangular prisms, or may have one or morefeatures that confer optical power to the article, such as curved faces,or microreplicated features, such as microlenses (and arrays thereof) orFresnel lenses. Further, the prisms may further comprise mirroredelements, such as a vapor deposited metal coating on or more surfaces.Combinations of these elements are also contemplated, for example acurved surface additionally having a diffractive element, such as amicrolens or microlens array, or a curved surface having a mirrorelement.

Although depicted as including two triangular prisms (see FIGS. 1 and2), the prisms may be any suitable shape disposed on one or both sidesof the polarizing layer to achieve the desired purpose of the PBS. Insome embodiments, one or more of the outer surfaces of the first andsecond prisms, i.e. one of the surfaces not adjacent the polarizinglayer, may be curved, either convex or concave, or may comprise astructured surface, such as a Fresnel lens surface. Such curved surfacesprovide optical power to the polarizing beam splitter; i.e., theyconverge or diverge light passing therethrough. The degree to which alens or mirror converges or diverges light usually is equal to thereciprocal of the focal length of the device.

Further, one or more of the first surfaces (i.e. the surface adjacent tothe polarizing layer) may be curved or microreplicated. For example, afirst prism may have a convex first surface, and a second prism may havea mating concave first surface, with a polarizing layer disposedtherebetween. Further, one or more of the outer surfaces of the firstand second prisms, (i.e. one of the surfaces not adjacent the polarizinglayer), may be fully or partially reflective; i.e. comprises avapor-deposited metal coating.

Reflective polarizing layers in exemplary PBS's constructed according tothe present disclosure include linear reflective polarizers having apass axis. In one embodiment, the polarizing layer may be a wire gridpolarizer, such as those described in Schnabel et al., “Study onPolarizing Visible Light by Subwavelength-Period Metal-Stripe Gratings”,Optical Engineering 38(2), pp. 220-226, February 1999, relevant portionsof which are hereby included by reference. A wire grid polarizerconsists of an array of very fine parallel lines or ribbons of metalcoated on glass or other transparent substrates. The wire arrayefficiently polarizes the incident light when the width and spacing aresmall compared to the incident wavelength(s). Common metals for the wiregrid array include gold, silver, and aluminum among others known in theart.

In one embodiment the polarizing beam splitter may comprise a firstprism having a first surface and at least two outer surfaces, a secondprism having a first surface and least two outer surfaces, and a wiregrid polarizer disposed between the first surfaces of the first andsecond prisms. Preferably, a wire grid polarizer comprising a substrate,such as glass, is bonded to the first surfaces by means of an opticaladhesive. Less preferably, the wire grid is deposited, such as by vapordeposition techniques, on one of said first surfaces, and the secondprism bonded thereto.

Wire-grid polarizers absorb small portions of the received light. Thisgenerates heat in the substrates and is therefore not preferred. Forexample, 5-10% of the light is absorbed by aluminum stripes in the samemanner as an aluminum mirror surface. Since the performance of thewire-grid polarizer is sensitive to the geometric stability of the metalstripes, a small change in the substrates due to thermal expansion candegrade the polarizer's performance.

In another embodiment, the polarizing layer may comprise alternatingrepeating layers of a pair of inorganic thin film materials deposited onthe first surface of one or both prisms. The pair of thin film materialscomprises one low refractive index material and one high refractiveindex material. The indices, called a MacNeille pair, are chosen suchthat, for a given angle of incidence of a light beam, the reflectioncoefficient for p-polarized light (rd is essentially zero at each thinfilm interface. The angle at which r_(p) is zero is called the Brewsterangle, and the formula relating the Brewster angle to the numericalvalues of the indices is called the MacNeille condition. The reflectioncoefficient for s-polarized light (r_(s)) is non-zero at each thin filminterface. Therefore, as more thin film layers are added, the totalreflectivity for s-polarized light increases while the reflectivity forp-polarized light remains essentially zero. Thus, an unpolarized beam oflight, incident upon the thin film stack, has some or all of thes-polarized components reflected while essentially all of thep-polarized component is transmitted.

In one embodiment, the repeating layers of a pair of inorganic thin filmmaterials (the optical stack) is deposited on the first surface of aprism and the bonded to the first surface of a second prism, such aswith an optical adhesive to form a polarizing beam splitter. Thepolarizing beam splitter comprises at least one set of pairs ofalternating layers of materials having low and high indices ofrefraction compared to each other. The thicknesses of the layers arechosen such that the quarterwave criterion is met for the wavelength ofthe incident collimated light beam by each of layers of low and highrefractive index material. The optical properties of the prism material,and the properties of the composite optical stack, all combine to dividethe incident light beam into two polarization components.

Suitable materials for the thin films include any materials that aretransparent (exhibit low absorption) in the spectrum of interest. Forbroadband visible light, suitable thin film materials are silicondioxide (n=1.45), amorphous hydrogenated silicon nitride (n=1.68-2.0);titanium dioxide (n=2.2-2.5) ; magnesium fluoride) (n=1.38); cryolite(Na₃AlF₆, n=1.35); zinc sulphide (n=2.1-2.4); zirconium oxide (n=2.05) ;haffnium oxide (n=2.0) ; and aluminum nitride (n=2.2).

Several thin film deposition techniques can be used to deposit thecomposite optical stack on the prisms, including thermal and electronbeam evaporation, and ion beam sputtering and magnetron sputtering.However, thermal and electron beam evaporation should provide good thinfilm qualities and sufficiently high deposition rates for acceptablemanufacturing rates. More importantly, low index films such as magnesiumfluoride and cryolite can be deposited by this method. Electron beamdeposition is regularly used in the coatings industry for high indexmaterials such as titanium dioxide, zirconium oxide, hafnium oxide, andaluminum nitride.

Preferably, the polarizing layer may be a multilayer optical film.Examples of reflective polarizing films suitable for use as polarizingfilm in the embodiments of the present disclosure include reflectivepolarizers including a birefringent material. manufactured by 3MCorporation, St. Paul, Minn., such as those described in U.S. Pat. No.5,882,774, (Jonza et al.); U.S. Pat. No. 6,609,795 (Weber et al.); andU.S. Pat. No. 6,719,426 (Magarill et al.), the disclosures of which arehereby incorporated by reference herein. Suitable reflective polarizingfilms for polarizing film 20 also include polymeric reflectivepolarizing films that include multiple layers of different polymericmaterials. For example, polarizing film may include a first layer and asecond layer, where the polymeric materials of the first and secondlayer are different and at least one of the first and second layers isbirefringent. In one embodiment of the present disclosure, thepolarizing film may include a multi-layer stack of first and secondalternating layers of different polymer materials, as disclosed in U.S.Pat. No. 6,609,795 (Weber et al.). Other materials suitable for makingmultilayer reflective polarizers are listed, for example in Jonza etal., U.S. Pat. No. 5,882,774; Weber et al., U.S. Pat. No. 6,609,795. Inanother embodiment of the present disclosure, multiple reflectivepolarizing films may be used.

Suitable reflective polarizing films are typically characterized by alarge refractive index difference between first and second polymericmaterials along a first direction in the plane of the film and a smallrefractive index difference between first and second polymeric materialsalong a second direction in the plane of the film, orthogonal to thefirst direction. In some exemplary embodiments, reflective polarizingfilms are also characterized by a small refractive index differencebetween the first and second polymeric materials along the thicknessdirection of the film (e.g., between the first and second layers ofdifferent polymeric materials). Examples of suitable refractive indexdifferences between the first and second polymeric materials in thestretched direction (i.e., x-direction) range from about 0.15 to about0.20. The refractive indices in the non-stretched directions (i.e., they-direction and the z-direction) are desirably within about 5% of oneanother for a given material or layer, and within about 5% of thecorresponding non-stretched directions of a different material or anadjacent layer.

The polymeric materials selected for the layers of an exemplarymultilayer polarizing film may include materials that exhibit low levelsof light absorption. For example, polyethylene terephthalate (PET)exhibits an absorption coefficient of less than 1.0×10⁻⁵ centimeter⁻¹.Accordingly, for a reflective polarizer film that includes PET and has athickness of about 125 micrometers, the calculated absorption is about0.000023%, which is about 1/200,000 of an absorption of a comparablewire-grid polarizer.

Low absorptions are desirable because polarizers used in PBS's areexposed to very high light density, which can lead to the failure of thepolarizers. For example, absorptive-type polarizer films absorb all thelight with unwanted polarization. This generates significant heat.Substrates with high thermal conductivity, such as sapphire, aretherefore needed to conduct the heat away from the polarizer films.Moreover, the substrates are exposed to high heat loads, whichcorrespondingly generate thermal birefringence in the substrates.Thermal birefringence in the substrates degrades the contrast andcontrast uniformity of the optical system, such as an image displaysystem. As a result, only few materials can be qualified for thesubstrates with traditional PBS's (e.g., sapphire, quartz, leads contentglass, and ceramics).

The present invention provides a multilayer article comprising amultilayer optical film and a cured optical coating on one or both majorsurfaces of the optical film. Providing such a coating protects themultilayer optical film from environmental stresses and adds strengthand rigidity thereto. The multilayer article may be prepared byproviding a multilayer optical film, coating at least one major surfaceof the multilayer optical film with the curable composition, and curing.In another embodiment, separately prepared films comprising the curedcomposition may be adhered to one or both major surfaces of themultilayer optical film by means of an optical adhesive, describedfurther herein.

The present invention provides a method of making a polarizing beamsplitter. The method comprises introducing the curable composition intoa suitable mold, and curing the composition to form a prism. The moldmay be of any suitable configuration, one or more surfaces of which maybe curved. The polarizing layer may then be bonded, adhered, orotherwise affixed to the resulting prism(s) by any optical adhesive,such as known in the art. In one embodiment, a first prism may be bondedto a first surface of the polarizing layer, the second prism bondedsequentially to the exposed surface of the polarizing layer. In anotherembodiment, the first and second prisms are concurrently bonded toopposite surfaces of the polarizing layer.

Useful optical adhesives are substantially free of UV-absorbingchromophores such as extended aromatic structures or conjugated doublebonds. Useful adhesives include, for example: NOA61, a UV curedthiol-ene based adhesive available from the Norland Company (Cranbury,N.J.); Loctite series (e.g., 3492, 3175) UV cured acrylic adhesivesavailable from Henkel Loctite Corp., Rocky Hill, Conn.; OP series (e.g.,21, 4-20632, 54, 44) UV cured acrylic adhesives available from DymaxCorporation, Torrington, Conn.

One useful adhesive include those compositions described in U.S.Published Appln. No. 20040202879 (Xia et al.), incorporated herein byreference, which comprise at least one polymer with either an acid orbase functionality that forms a pressure sensitive adhesive, a highT_(g) polymer with an weight average molecular weight greater than100,000 with an acid or base functionality, and a crosslinker, whereinthe functionality on the pressure sensitive adhesive and the high T_(g)polymer cause an acid-base interaction that forms a compatibilizedblend. After accelerated aging of the adhesive composition at 80° C. and90% relative humidity for approximately 500 hours in an oven, theadhesive mixture is translucent or optically clear.

Another useful adhesive includes microstructured adhesive, whichcomprise a continuous layer of a pressure-sensitive adhesive having amicrostructured surface, wherein the microstructured surface comprises aseries of features and wherein the lateral aspect ratio of the featuresrange from about 0.1 to about 10, wherein the spacing aspect ratio ofthe features range from about I to about 1.9, and wherein each featurehas a height of about 2.5 to about 375 micrometers. Such adhesives aredescribed in U.S. Pat. Nos. 5,650,215, 6,123,890, 6,315,851, 6,440,880and 6,838,150 (each Benson et al.) incorporated herein by reference.

Other useful adhesives include Soken™ 1885 PSA (commercially availablefrom Soken Chemical & Engineering Co., Ltd, Japan), NEA PSA (asdescribed in the Example 1 of published U.S. 20040202879 (Lu et al.)),Lens Bond™ Type C59 (a thermally cured styrene based adhesive availablefrom Summers Optical, Hatfield, Pa., a division of EMS AcquisitionCorp., and NOA61™ (a UV cured thiol-ene based adhesive, available fromNorland Company, Cranbury, N.J.).

In another embodiment, the polarizer may be prepared as shownschematically in FIG. 1. Here, a prism mold 10 a, having an open firstsurface or face, and optional tabs 11 a and b, is combined with apolarizing layer 20 and rigid side plate 30. The angles between the moldfaces may be varied as desired, and either or both outer faces 12 a/bmay be curved or have any desired pattern imparted thereto, such as adiffracting pattern, such as a Fresnel lens may be integrally molded.The respective first surfaces of the first and second prisms may also becurved, or have an integral microreplicated pattern. Advantageously, thecurved first surfaces of the first and second prisms may configured sothey may be mated, such as with a concave and convex surface, with thepolarizing layer disposed therebetween.

The parts 10 a, 20 and 30 may be secured via clamps on tabs 11 a/b, orby other suitable means. A tensioning means (not shown) may be used tomaintain the polarizing layer flat. The assembled mold may rest on asmooth, rigid surface 15, such as glass, or an integral bottom may beprovided to the mold 10 a. This assembly defines a prism shaped cavity40 a, into which the curable composition may be introduced, and cured byapplication of UV energy. If desired, a second rigid surface (not shown)may cover the top of the mold to protect it from atmospheric oxygen.Desirably, either rid surface 15 or second rigid surface is made ofglass or other suitable material transparent to the incident lightsource for curing. Alternatively to the second rigid surface, the moldassembly may be blanketed with an inert glass to exclude oxygen.

Upon cure, the rigid side plate 30 may be removed, and a second prismmold 10 b secured thereto, forming a second chamber 40 b with thepolarizing layer 20 forming the common faces between molds 10 a and 10b, so that the first surfaces of the first and second prisms will eachbe adjacent the polarizing layer As result of the curing the polarizinglayer is now integral to the first surface of the first prism-shapedcured composition. The second mold may also have curved outer faces orother desired molded shapes (not shown). This second chamber 40 b may befilled with the curable composition, cured, the mold assembly removed toprovide a polarizing beam splitter 60 a having two prisms, and anintegral polarizing layer disposed therebetween, on the respective firstsurfaces of the prisms. If desired, the respective prisms may beprovided, by a suitably configured mold, with integral interlocking tabsfor securing the first and second prisms together or may comprise tabsfor securing the beam splitter into a mount in a display device.Further, the first and/or second prisms may be providing with alignmentmeans, such as tabs or indicators, for aligning the first and secondprisms with respect to each other, the polarizing layer, and a mount ina display device.

The alignment means may comprise corresponding male and female portionsthat interconnect. The polarizing beams splitter may comprise a firstprism and second prism, where first prism includes male portion, andsecond prism that includes female portion. Male portion may be arectangular surface that encompasses a portion of the surface of firstprism adjacent to the reflective polarizing film, and which may projecttherefrom. Similarly, female portion may be a rectangular depressionthat is disposed within the majority of the surface of second prismadjacent to the reflective polarizing film.

The male members and female portions may be substituted with otherengagement mechanisms such that one prism includes at least one malemember that is configured to engage with a respective female portionlocated in the opposing prism. Those of ordinary skill in the art willalso readily appreciate that different numbers of the male members andthe female portions than those exemplified herein may be used inaccordance with the present disclosure. For example, an exemplary PBSmay include three or more male members received within three or morefemale portions.

The male members and the female portions discussed above may be moldedwith the respective first and second prisms. The first and second prismsmay then be secured together with the assistance of the male portionsand the female portions to form polarizing beam splitters. Thistechnique may involve placing the reflective polarizing film between thefirst prism and the second prism. The first prism may then be orientedrelative to the second prism such that the male portion(s) are alignedwith the corresponding female portion(s). This alignment is beneficialfor ensuring that the first prism is accurately positioned relative tothe second prism. The first prism may then engage second prism byconcurrently inserting male portions into the corresponding femaleportions. This compresses the reflective polarizing film between theincident surfaces of the first prism and the second prism to provide asmooth, planar interface. The male portion(s) may be secured to thecorresponding female portions with an adhesive. Additionally, the firstprism may be secured to the second prism by fitting and/or welding themale members to the corresponding female portions (e.g., ultrasonic,infrared, heat staking, snap fits, press fits, and chemical welding).

An alternate process is shown schematically in FIG. 2. Here, two prismmolds having open faces, 110 a and 120 b (corresponding to the firstsurfaces of the resultant prisms) are secured together via optional tabs111 a and b (or any suitable means), with the polarizing layer 120disposed between on the common first surfaces of molds 110 a and b. Thiscreates two chambers 140 a/b that may be filled with the curablecomposition, and cured to produce a polarizing beam splitter in a singlestep. Again, the molds may be of any suitable shape and size, and theexterior faces may be curved.

EXAMPLES

These examples are merely for illustrative purposes only and are notmeant to be limiting on the scope of the appended claims. All parts,percentages, ratios, etc. in the examples and the rest of thespecification are by weight, unless noted otherwise. Solvents and otherreagents used were obtained from Sigma-Aldrich Chemical Company;Milwaukee, Wis. unless otherwise noted. Table of AbbreviationsAbbreviation or Trade Designation Description IBOA Isobornyl acrylate,available from Sartomer Company Inc, Exton, PA MMA Methyl methacrylateHEA Hydroxyethyl acrylate HEMA Hydroxyethyl methacrylate IOTG Isooctylthioglycolate, available from TCI America, Portland, OR MCEMercaptoethanol IEM Isocyanatoethyl methacrylate, available from ShowaDenka, Japan MAnh Methacrylic anhydride HDDMA 1,6 Hexanedioldimethacrylate, SR239, available from Sartomer Company Inc, Exton, PAVazo 52 2,2′-Azobis(2,4-dimethylvaleronitrile), available from DuPontCompany, Wilmington, DE Vazo 88 1,1′-Azobis(cyanocyclohexane), availablefrom DuPont Company, Wilmington, DE DBDL Dibutyltin dilaurate A31Release A Silicone liner from DuPont TeiJin Films U.S. Liner LimitedPartnership, Wilmington, DE Lucirin TPO-L Ethyl 2,4,6-trimethylbenzoylphenyl phosphinate available from BASF, Mt. Olive, NJ CN945A60Trifunctional aliphatic urethane acrylate blended with SR306,(tripropyleneglycol diacrylate), in an approximate 60:40 ratio availablefrom Sartomer Company Inc, Exton, PA CN1963 Aliphatic urethanedimethacrylate blended with TMPTMA (trimethylolpropane trimethacrylate),in an approximate 75:25 ratio available from Sartomer Company Inc,Exton, PA PBS FILM multilayer reflective polarizing film manufactured by3M Corporation, St. Paul, MN, described in U.S. Pat. No. 5,882,774,(Jonza et al.); U.S. Pat. No. 6,609,795 (Weber et al.); and U.S. Pat.No. 6,719,426 (Magarill et al.).

Test Methods

Molecular Weight Measurement by SEC

Size exclusion chromatography (SEC) for molecular weight and molecularweight distribution was performed using a Waters 717Plus autosampler,1515 HPLC pump, 2410 differential detector, and the following Waterscolumns: Styragel HR 5E, Styragel HR 1. All samples were run in THF at35° C. with a flow rate of 1.0 mL/min. Linear polystyrene standards wereused for calibration.

Dynamic Mechanical Analysis (DMA) Measurement

DMA for Tg and modulus determination of cured compositions was performedusing a LC-ARES Test Station (Rheometric Scientific, Piscataway, N.J.)in a torsion mode. The sample size was approximately 25 millimeters by10 millimeters by 1 millimeter. The length of the sample was measured bythe test station and the width and thickness of the sample were measuredwith a caliper. The test was performed by ramping the temperature from25° C. to 180° C. at 5° C. per minute. The frequency used was 1 Hertz.

Yellowing Resistance Test

The % Transmittance (% T) at a wavelength of 420 nanometers of 3.2centimeter (1.25 inches) diameter by 0.5 centimeter (0.19 inch) thickcured samples was measured before and after 7 days aging in a 120° C.oven. The % T was measured using a TCS Plus Spectrophotometer(BYK-Gardner USA, Silver Spring, Mo.). Generally, samples with % T at420 nanometers of less than 85% display a yellow color. A sample isconsidered to have good yellowing resistance if the % T at 420nanometers after aging is greater than 85%.

Volume Shrinkage Determination

Density of the curable compositions and the cured materials weremeasured by a pycnometer. % volume shrinkage was calculated based on thedensity change during cure of the curable materials. % volumeshrinkage=100×(density of cured material−density of curable materialbefore cure)/density of curable material before cure.

Water Absorption Measurement

A weighed 3.2 centimeter (1.25 inches) diameter by 0.5 centimeter (0.19inch) thick cured disk sample is placed in water at 23° C. for 14 days.% water absorption=100×(sample weight after 14 days in water−sampleweight before water soaking)/sample weight before water soaking.

Birefringence Determination

Transmission Spectral Ellipsometry (TSE) was used to measure retardanceof the sample. Birefringence of the sample was determined by dividingthe retardance by sample thickness. The sample is a round disc, 3.2centimeter (1.25 inches) diameter by 0.5 centimeter (0.19 inch) thick.The sample was mounted on a rotating stage, and the TSE retardance datawas measured at a series of positions using a J. A. Woollam M2000Variable Angle Spectral Ellipsometer. In-plane measurements were takenat 4 locations 6 millimeters apart in two orthogonal directions, for atotal of 8 in-plane measurements. The measured retardances were averagedin the wavelength range between λ=545-555 nanometers.

Examples 1-7

Preparation of Oligomer Syrups:

In Examples 1-7, IBOA, HEA, chain transfer agent IOTG or MCE, and the1^(st) charge of thermal initiators Vazo 52 and 88, according to Table1, were added to a four neck flask equipped with a reflux condenser,thermometer, mechanical stirrer, and nitrogen gas inlet. The mixture wasstirred and heated to 60° C. under nitrogen. The temperature of thereaction mixture peaked at around 150° C. during the polymerization.After the reaction peak, the batch was further polymerized at 140° C.for 30 minutes with the addition of the 2^(nd) initiator, Vazo 88, toreduce residual monomers and eliminate initiator. A sample was taken atthe end of this reaction period for oligomer molecular weightdetermination by SEC. After that, the batch was cooled to 100° C. TheHDDMA reactive diluent was added to the reactor to reduce viscosity ofthe batch. A solution of the DBDL catalyst in IEM was then added to thebatch to react with the hydroxyls on the IBOA/HEA polymer chains,incorporating methacrylate functional groups to the polymer. Thereaction was complete in 2 hours.

Preparation of Cured Samples:

After completion of the reaction, the reactive oligomer syrups of Table1 were formulated with 0.02 weight % TPOL photoinitiator and cured by aXenon Flash lamp according to the procedures described in the sectionPreparation of Test Samples. The cured samples were tested for % volumeshrinkage, birefringence, Tg, water absorption, % transmittance, andaging stability, using the methods described in the above Test Methodsection. TABLE 1 Example 1 2 3 4 5 6 7 IBOA* (g) 190.0 190.0 190.0 190.0180.0 180.0 190.0 HEA (g) 10.0 10.0 10.0 10.0 20.0 20.0 10.0 IOTG (g)1.0 2.0 4.0 16.0 16.0 — 4.0 MCE (g) — — — — — 20.0 — Vazo 52/(g) 0.025/0.025/ 0.025/ 0.025/ 0.025/ 0.025/ 0.025/ Vazo 88 (g) 0.025 0.025 0.0250.025 0.025 0.025 0.025 Vazo 88 (g) 0.050 0.050 0.050 0.050 0.050 0.0500.050 HDDMA* (g) 66.7 50.0 50.0 22.2 22.2 22.2 22.2 IEM* (g) 13.36 13.3613.36 13.36 26.7 53.4 13.36 DBDL (g) 0.0467 0.0467 0.0467 0.0467 0.0930.18 0.0467*The IBOA monomer and HDDMA reactive diluent in Examples 3, 4 and 5 werepurified by passing through a column of activated basic aluminum oxidepowder, Brokmann1, ˜150 mesh, 58 A°, 155 m²/g, from Aldrich to removeinhibitors. The IEM monomer in the same examples was distilled to removethe inhibitor. The other ingredients in Examples 3, 4, and 5, and allingredients in Examples 1, 2, 6, and 7 were used as received from thevendors without purification.

Examples 8-10

To prepare the reactive oligomer syrups of Examples 8-10, HDDMA reactivediluent, according to Table 2, was added to samples of the reactiveoligomer syrup prepared in Example 7. The reactive oligomer syrups wereformulated with 0.02 weight % TPOL photoinitiator and cured by a XenonFlash lamp at 80° C. for 5 minutes. The cured samples were tested for %volume shrinkage, birefringence, Tg, water absorption, % transmittance,and aging stability, using the methods described in the above TestMethod section. TABLE 2 Example 8 9 10 Oligomer/HDDMA Ratio 80/20 70/3060/40 Example 7 (grams) 20.0 20.0 20.0 HDDMA (grams) 2.50 5.71 10.0 TPOLphotoinitiator (grams) 0.0045 0.0051 0.0060

Examples 11-12

To prepare reactive oligomer syrups of Example 11 and 12 in Table 3, thesame polymerization procedure described in the Examples 1-7 above wasfollowed, except 30 minutes after the addition of the 2^(nd) initiatorat 140° C., MAnh was added and the mixture was reacted at 140° C. foranother 3 hours with efficient stirring. After that, the batch wascooled down to 110° C. and HDDMA reactive diluent was added to thereactor to further reduce the viscosity of the batch. TABLE 3 Example 1112 IBOA (g) 190 160 MMA (g) 0 26 HEA (g) 10 10 HEMA (g) 0 4 IOTG (g) 4 8Vazo 52/(g) 0.025/ 0.025/ Vazo 88 (g) 0.025 0.025 Vazo 88 (g) 0.0500.050 MAnh (g) 14.12 19.16 HDDMA (g) 50 35.3

Examples 13-14

The reactive oligomer and reactive diluent mixtures, CN945A60, CN1963were formulated with 0.02% TPOL photoinitiator and cured by a XenonFlash lamp at 80° C. for 5 minutes according to procedures described inthe preparation of test sample. TABLE 4 Example 13 14 CN945A60 (g) 100 —CN1963 (g) — 100 TPOL photoinitiator (g) 0.02 0.02Preparation of Test Samples of Examples 1-14

Curable compositions with photoinitiator and other additives wereprepared by preheating the oligomer syrups described in the Examples andComparative Examples with a desired photoinitiator and other additives(if used) at 80° C. and mixing in a white disposable cup by a DAC-100mixer (both cup and mixer are available from Flack Tek Inc, Landrum,N.J.). The compositions were degassed in a vacuum chamber and thenallowed to cool to room temperature before use.

Curing of the above curable materials was carried out by the followingsteps: 1) Onto a Pyrex glass plate approximately 15 centimeters (6inches) by 15 centimeters (6 inches) by 0.5 centimeter (0.19 inch) wasplaced an approximately 15 centimeters (6 inches) by 15 centimeters (6inches) piece of 51 micrometers (2 mils) A31 release liner, 2) on top ofthe release liner was placed an approximate same size glass or siliconerubber mold with a 3 centimeter (1.25 inches) diameter opening at thecenter, 3) then the mold was filled with the curable compositions takingcare to avoid bubbles, 4) then a second piece of approximately 15centimeters (6 inches) by 15 centimeters (6 inches) of 51 micrometers (2mils) A31 release liner was placed on top of the filled mold, 5) anotherPyrex glass plate approximately 15 centimeters (6 inches) by 15centimeters (6 inches) by 0.5 centimeter (0.19 inch) was placed on topof the release liner, and 6) finally, the filled mold was placed onto aheating station at 80° C. in a chamber and allowed to equilibrate. Thecurable compositions were cured by a Xenon flash lamp (Model #4.2 LampHsg, pulse rate of 8 Hz) with RC-747 Pulsed UV/Visible System (XenonCorporation, Woburn, Mass.) for 5 minutes.

The weight average molecular weight which was measured by SEC, the Tgwhich was measured by Dynamic Mechanical Analysis (DMA), the % volumeshrinkage which was determined by a pycnometer, and the birefringencewhich was measured by wellipsometry are shown in Table 5. Visiblynoticeable color and % transmittance measured by the TCS PlusSpectrophotometer after 7 days aging in a 120° C. oven are also shown inTable 5. Water absorption data obtained as described in the Test Methodsection above are shown Table 6. TABLE 5 Sample Color % T at 420 nmVolume after 120° C. after 120° C. Oligo. Oligo/ Tg Shrinkage aging for7 aging for 7 Example MW HDDMA (%) (° C.) (%) Birefringence days days 130K   75/25 — — 2.59 × 10⁻⁷ None 87.3 2 18K   80/20 — — 1.51 × 10⁻⁷ None89.3 3 9.0K 80/20 109 2.8 5.07 × 10⁻⁷ — — 4 3.0K 90/10 — 2.1 1.32 × 10⁻⁷— — 5 3.2K 90/10 88 2.4 4.03 × 10⁻⁷ — — 6 1.4K 90/10 — — 6.33 × 10⁻⁷ — —7 9.0K 90/10 94 0.9 1.82 × 10⁻⁷ None 89.9 8 9.0K 80/20 100 2.7 1.47 ×10⁻⁷ None 90.2 9 9.0K 70/30 94 4.7 4.39 × 10⁻⁷ None 90.1 10 9.0K 60/4079 5.5 1.24 × 10⁻⁶ None 89.4 11 9.0K 80/20 — — — None 91.3 12 6.5K 85/15— — — None 90.5 13 — — 53 — — Yellow 81.0 14 1.2K — 97 6.8 3.88 × 10⁻⁶Yellow 74.3 (3 days)

TABLE 6 Water Absorption Example (14 days in 23° C. water) 8 0.17% 110.22% 14 0.87%

Example 15

In Examples 15, IBOA, HEA, chain transfer agent IOTG, and the 1^(st)charge of thermal initiators Vazo 52 and 88, according to Table 7, wereadded to a four neck flask equipped with a reflux condenser,thermometer, mechanical stirrer, and nitrogen gas inlet. The mixture wasstirred and heated to 60° C. under nitrogen. The temperature of thereaction mixture peaked at around 180° C. during the polymerization.After the reaction peak, the batch was further polymerized at 140° C.for 30 minutes with the addition of the 2^(nd) initiator, Vazo 88, toreduce residual monomers and eliminate initiator. After that, the batchwas cooled to 120° C. and the IBOA reactive diluent was added to thereactor to reduce viscosity of the batch. MAnh was then added to thebatch to react with the hydroxyl groups on the oligomer chains toincorporate methacrylate groups. The reaction time was approximately 6hours. The HDDMA was then added and the solution was cooled to ambient.The reactive oligomer syrups of Table 7 were formulated with 0.02 weight% Lucirin TPOL photoinitiator to make the curable material compositionto prepare plastic prisms and PBS prisms. TABLE 7 Example 15 IBOA (g)185 HEA (g) 15 IOTG (g) 8 Vazo 52 (g)/Vazo 88 (g) 0.025/0.025 Vazo 88(g) 0.050 IBOA (g) 10 MAnh (g) 21.18 HDDMA (g) 35.3 Lucirin TPOL (g)0.055

Example 16 Preparation of a Plastic Prism

For Example 16 the prism mold described in FIG. 1 was used. Component 10a was made of stainless steel with component 15 being a glass plate,component 20 was not used and component 30 was a glass microscope slide.The volume 40 a was filled with the formulated reactive oligomer syrupprepared in Example 15 and another glass plate was placed on top offilled volume 40 a. The assembly was cured with a Xenon flash lamp atroom temperature for 5 minutes.

Example 17 Preparation of a Plastic PBS Prism

For Example 17, the prism mold described in FIG. 1 was used. Component10 a was made of stainless steel with component 15 being a glass plate,component 20 was PBS FILM and component 30 was a glass microscope slide.The volume 40 a was filled with the formulated reactive oligomer syrupprepared in Example 15 and another glass plate was placed on top offilled volume 40 a. The assembly was cured with a Xenon flash lamp atroom temperature for 5 minutes. The glass slide was removed and aplastic prism such as prepared in Example 16 was attached to the PBSFILM surface with the formulated reactive oligomer syrup prepared inExample 15 used as an optical adhesive to generate a PBS prism. Theadhesive was cured with a Xenon flash lamp at room temperature for 5minutes.

Example 18 Preparation of a Plastic PBS Prism

For Example 18, the prism molds described in FIG. 1 were used. Component10 a was made of stainless steel with component 15 being a glass plate,component 20 was PBS FILM and component 30 was a glass microscope slide.The volume 40 a was filled with the formulated reactive oligomer syrupprepared in Example 15 and another glass plate was placed on top offilled volume 40 a. The assembly was cured with a Xenon flash lamp atroom temperature for 5 minutes. The component 30 was then removed and asecond mold 10 b was placed adjacent to the cured mold. The volume 40 bwas filled with the formulated reactive oligomer syrup prepared inExample 15 and a glass plate was placed on top of filled volume 40 b.The assembly was cured with a Xenon flash lamp at room temperature for 5minutes.

Example 19 Preparation of a Plastic PBS Prism

For Example 19, the prism molds described in FIG. 2 were used.Components 110 a and 110 b were made of stainless steel with component115 being a glass plate, component 120 was PBS FILM. The volumes oneither side of 120 were filled with the formulated reactive oligomersyrup prepared in Example 15 and another glass plate was placed on topof the filled volumes. The assembly was cured with a Xenon flash lamp atroom temperature for 5 minutes.

Example 20 Preparation of a Plastic Prism

For Example 20 the prism mold described in FIG. 1 was used. Component 10a was made of stainless steel with component 15 being a glass plate,component 20 was not used and component 30 was a glass microscope slide.The volume 40 a was filled with the formulated reactive oligomer syrupprepared in Example 14 and another glass plate was placed on top offilled volume 40 a. The assembly was cured with a Xenon flash lamp atroom temperature for 5 minutes.

Example 21 Preparation of a Plastic PBS Prism

For Example 21, the prism mold described in FIG. 1 was used. Component10 a was made of stainless steel with component 15 being a glass plate,component 20 was PBS FILM and component 30 was a glass microscope slide.The volume 40 a was filled with the formulated reactive oligomer syrupprepared in Example 14 and another glass plate was placed on top offilled volume 40 a. The assembly was cured with a Xenon flash lamp atroom temperature for 5 minutes. The glass slide was removed and aplastic prism such as prepared in Example 20 was attached to the PBSFILM surface with the syrup prepared in Example 14 used as an opticaladhesive, to generate a PBS prism. The adhesive was cured with a Xenonflash lamp at room temperature for 5 minutes.

Example 22 Preparation of a Plastic PBS Prism

For Example 22, the prism molds described in FIG. 1 were used. Component10 a was made of stainless steel with component 15 being a glass plate,component 20 was PBS FILM and component 30 was a glass microscope slide.The volume 40 a was filled with the formulated reactive oligomer syrupprepared in Example 14 and another glass plate was placed on top offilled volume 40 a. The assembly was cured with a Xenon flash lamp atroom temperature for 5 minutes. The component 30 was then removed and asecond mold 10 b was placed adjacent to the cured mold. The volume 40 bwas filled with the formulated reactive oligomer syrup prepared inExample 14 and a glass plate was placed on top of filled volume 40 b.The assembly was cured with a Xenon flash lamp at room temperature for 5minutes.

Example 23 Preparation of a Plastic PBS Prism

For Example 23, the prism molds described in FIG. 2 were used.Components 110 a and 110 b were made of stainless steel with component115 being a glass plate, component 120 was PBS FILM. The volumes oneither side of 120 were filled with the formulated reactive oligomersyrup prepared in Example 14 and another glass plate was placed on topof the filled volumes. The assembly was cured with a Xenon flash lamp atroom temperature for 5 minutes.

1. An optical article comprising a cured composition comprising a) 50 to99 parts by weight of an oligomer having a plurality of pendent,free-radically polymerizable functional groups and a T_(g) of ≧20° C.;b) 1 to 50 parts by weight of a free-radically polymerizablecrosslinking agent and/or a diluent monomer, and c) 0.001 to 5 parts byweight of a photoinitiator, based on 100 parts by weight of a) and b).2. The optical article of claim 1 selected from the groups consisting ofoptical lenses, optical fibers, prisms, diffractive lenses, microlenses,microlens arrays, Fresnel lenses, light guides, wave guides and opticalfilms.
 3. The optical article of claim 1 wherein said oligomer isselected from poly(meth)acrylate, polyurethane, polyepoxide, polyester,polyether, polysulfide, and polycarbonate oligomers.
 4. A polarizingbeam splitter comprising: a first prism having a first surface and atleast two outer surfaces, and a second prism having a first surface andat least two outer surfaces, and an polarizing layer having a pass axisdisposed between said first surfaces of the first and second polarizers,wherein at least one prism comprises a cured composition comprising a)50 to 99 parts by weight of an oligomer having a plurality of pendent,free-radically polymerizable functional groups and a T_(g) of ≧20° C.;b) 1 to 50 parts by weight of a free-radically polymerizablecrosslinking agent and/or a diluent monomer, and c) 0.001 to 5 parts byweight of a photoinitiator, based on 100 parts by weight of a) and b).5. The polarizing beam splitter of claim 4, wherein said polarizinglayer comprises a multilayer birefringent film.
 6. The polarizing beamsplitter of claim 4 wherein said polarizing layer comprises a multilayerbirefringent film layer.
 7. The polarizing beam splitter of claim 6wherein said multilayer birefringent film is adhesively bonded to thefirst surfaces of the first and second prisms.
 8. The polarizing beamsplitter of claim 6 wherein said multilayer birefringent film isintegral to the first surfaces of the first and second prisms.
 9. Thepolarizing beam splitter of claim 4, wherein the birefringent filmcomprises a plurality of alternating first polymer layers having a highindex of refraction and a plurality of second polymer layers having alower index of refraction.
 10. The polarizing beam splitter of claim 9wherein adjacent pairs of layers of the birefringent film have a totaloptical thickness that is ½ of the wavelength of the light desired to bereflected.
 11. The polarizing beam splitter of claim 9 wherein adjacentpairs of layers have a total optical thickness that is ¼ of thewavelength of the light desired to be reflected.
 12. The polarizing beamsplitter of claim 4 wherein the prisms exhibit a birefringence of lessthan 1×10⁻⁶.
 13. The polarizing beam splitter of claim 4 wherein atleast one of said outer surfaces of either the first and second prismshas optical power.
 14. The polarizing beam splitter of claim 13 whereinthe surface having optical power comprises an integral Fresnel lens. 15.The polarizing beam splitter of claim 13 wherein the surface havingoptical power is curved.
 16. The polarizing beam splitter of claim 15wherein the curved surface is a convex or concave curved surface. 17.The polarizing beam splitter of claim 4 wherein the first surfaces ofthe first and second prisms are mating curved surfaces, with thepolarizing layer disposed therebetween.
 18. The polarizing beam splitterof claim 4, wherein one of more outer surfaces of the first and secondprisms are mirrored.
 19. The polarizing beam splitter of claim 4 whereinthe first and/or second prisms have an alignment means.
 20. The opticalarticle of claim 1 wherein said oligomer comprises polymerized monomerunits comprising: a) 50 to 99 parts by weight of (meth)acryloyl monomerunits homopolymerizable to a polymer having a glass transitiontemperature≧20° C., b) 1 to 50 parts by weight of monomer units having apendent, free-radically polymerizable functional group c) less than 40parts by weight of monomer units homopolymerizable to a polymer having aglass transition temperature<20° C., based on 100 parts by weight of a)and b)
 21. A method of making the polarizing beam splitter of claim 4comprising the steps of providing a first and second prisms comprisingthe cured oligomer composition, each prism having a first surface and atleast two outer surfaces, and adhesively securing a polarizing layerbetween the first surfaces of the first and second prisms.
 22. A methodof making the polarizing beam splitter of claim 4 comprising the stepsof providing a prism mold, having an open first face, securing apolarizing layer to the open face to form a prism mold chamber, castingsaid chamber with the curable oligomer composition, curing thecomposition, and securing a second prism to the polarizing layer. 23.The method of claim 21, wherein said second prism is adhesively securedto the polarizing layer.
 24. The method of claim 21 wherein said secondprism is secured to the polarizing layer by providing a second prismmold having an open face secured to the first mold, then casting andcuring the curable oligomer composition.
 25. The polarizing beamsplitter of claim 4 wherein at least one outer surface comprises adiffractive element.
 26. A method of making the polarizing beam splitterof claim 4 comprising the steps of providing a first and second prismmold, having the polarizing layer disposed therebetween, casting andcuring the curable composition in said molds, to produce a polarizingbeam splitter having an integral polarizing layer.